Biomedical Engineering Reference
In-Depth Information
resistant owing to the presence of an extremely thin passive oxide film (<
7 nm)
which spontaneously forms on their surfaces, even in solutions with
low oxygen contents, and which serves as a physical barrier against the
action of the aggressive fluids. According to Marcus
41
the role of alloying
elements in the passivation of alloys is based on the metal-oxygen and
the metal-metal bond strengths. alloying elements with a high energy of
adsorption of oxygen and low metal-metal bond energy are predicted to
enhance passivity. For instance, the relatively high energy of adsorption of
oxygen (∆
H
ads
= 737 kJ mol
-1
) and the low Cr-Cr bond strength (∆
H
B
= 99.1
kJ mol
-1
) allow the easy breaking of the metal-metal bond, favouring oxide
nucleation. alloying elements that have a high metal-metal bond strength
cause a lowering of the dissolution rate by increasing the activation energy
barrier for the disruption of the metal-metal bonds in the alloy surface, thus
they are so-called dissolution blockers.
Once the oxide passive film is formed, the corrosion behaviour depends to
a large extent on its structure and chemistry, which are themselves dependent
on the thermal, mechanical and electrochemical history. Passivation of stainless
steel and Co-base alloys lie in the formation of Cr-rich oxide films, whereas
Ti-base alloys yield to the formation of outer Ti-rich oxide films. In recent
years there has been increasing interest in the nature of the oxide developed
on the surface of Ti and Ti-based alloys and its biological significance. It has
been shown by scanning auger Microscopy that composition of the passive
film on the Ti-6Al-7Nb is heterogeneous and reflects the underlying a+b
microstructure of the substrate.
42
The oxide layer developed on the a phase
is enriched in al by a factor of two, while the oxide above the b phase has
a sixfold higher nb content. These compositional differences were found to
point to small variations in surface charges.
The most corrosion-resistant materials are titanium and its alloys, followed
by wrought and cast cobalt base alloys. Type 316L is the least corrosion
resistant of the implant metals.
43
Passivating steel in a 20-40 vol% nitric acid
solution at 60ºC for 30 minutes removes surface defects which could serve
as pit sites and improves their corrosion behaviour. The same ranking can
be observed during the measurement of polarization resistance. Corrosion
takes place by several mechanisms that may occur alone (pitting, crevice,
galvanic, intergranular) or be assisted by external loads (stress-corrosion
cracking, corrosion fatigue and fretting corrosion).
44
Pitting corrosion refers to a severe form of localized attack that results
in the release of significant amounts of metal ions. It is manifested by the
presence of cavities at the surface. Pits may be initiated at surface defects in
the material or passive film. Their formation is attributed to the interaction
of aggressive ions, such as Cl
-
, with the passive film. Once the passive film
is broken, the forming pit becomes covered by precipitates that restrict the
entry of the solution and oxygen into the pit, making repassivation impossible.
